Saturable absorption elements are capable of controlling transmittance or refractive index of signal light by irradiating or not irradiating control light, and are usable in optical gate switches, optical AND devices, demultiplexers (DMUX) of time division multiplexed signal, optical modulators, wavelength converters, and so forth. Hereinbelow, these devices are called optical gate switches, etc.
Absorption saturation effect of a semiconductor pertains to the nature that, when light of an energy just above a bandgap energy enters into the semiconductor, the absorptance (transmittance) non-linearly decreases (increases) as the incident light intensity increases, and it occurs due to the band filling effect where electron-hole pairs generated by absorption increase the state occupation factor within a band and the absorption end shifts toward a higher energy side. Normally, therefore, signal light does not pass through due to large absorption. However, when control light of an intensity over a certain value enters and causes absorption saturation of a saturable absorption element, the transmittance increases and permits the signal light to pass through. As a result, The saturable absorption element can be used as an optical gate switch, etc.
Changes in transmittance cause changes in refractive index according to the Kramas-Kronig relation. Therefore, saturable absorption elements are characterized in being variable in refractive index in response to the intensity of incident light. Since changes in refractive index causes the phase of the signal light to change, an optical gate switch, etc. having a function similar to the changes in absorptance can be realized by using interference of two optical paths.
However, observing the changes in transmittance with time when control pulse light enters into a semiconductor saturable absorption element, although the absorptance decreases with a sufficiently quick response on the order of picoseconds at the rise of the control pulse light, the time required to the absorptance to return the original value after the fall of the control pulse light is as long as the lifetime of carriers (electrons and holes) approximately. This aspect is schematically shown in FIGS. 12(a) through 12(c) in which FIG. 12(a) shows changes in intensity of control pulse light with time, FIG. 12(b) shows changes in absorption coefficient of a saturable absorption element with time relative to signal light, and FIG. 12(c) shows changes in refractive index, putting time (ps) on the abscissa. As shown here, even when the control pulse light turns out OFF, it takes approximately 1 nanosecond for the absorption coefficient to recover. Since the absorptance recovery time is restricted by the so-called carrier lifetime, gating action could not follow fast-cyclic control pulse light as short as the carrier lifetime or less.
For the purpose of reducing the carrier lifetime of a semiconductor saturable absorption element, there was a proposal to introduce impurities or defects into the absorbing layer to shorten the carrier lifetime to sub-picoseconds. However, since the process of recombination is non-radiative recombination, and the energy by recombination is converted to a heat energy, it is not suitable for fast-cyclic repetition. Further proposals were also made, such as applying a reverse bias to remove carriers, removing carriers generated in the absorbing region by utilizing tunneling effect, but they could not realize quickness sufficiently responsive to optical pulses on the order of picoseconds.
As explained above, conventional saturable absorption elements had the drawbacks that, once they got into absorption saturation, the time required for the absorptance or refractive index to return the original value was limited by the carrier lifetime. Therefore, in a gate element using such a saturable absorption element directly, the gate action could not follow fast-cyclic control pulse light as short as the carrier lifetime or less.